1. Field of the Invention
The present invention relates to pneumatic tires improved in wet grip performance without deteriorating steering stability.
2. Description of the Background Art
For example, as shown in FIG. 6, there is suggested a tire having crown main grooves b1 arranged on both sides of a tire equator Co and shoulder main grooves b2 arranged axially outward from the crown main grooves b1, whereby a tread section a is divided into a crown rib c1, middle ribs c2 and c2, and shoulder ribs c3 and c3, and the ribs c1, c2 and c3 are provided with lateral grooves d1, d2, and d3 (refer to Japanese Patent Application Publication No. 2004-17863, for example).
This tire includes the lateral grooves d2 which are arranged on the middle ribs c2, inclined at an angle θ of 10 to 45 degrees with respect to the tire circumferential direction, and are formed as steeply inclined grooves e with the angle θ decreased gradually axially inward. Such a tire cuts off water films on roads and delivers excellent drainage performance approximate to that of a tire with circumferential grooves, thereby achieving excellent wet grip performance. In addition, the inclined grooves e have axial inner ends discontinued near the crown main grooves b1 without intersecting the same, which suppresses reduction in pattern rigidity to thereby provide steering stability on dry roads.
In recent years, however, with upgrading of road networks and increase in speed and performance of vehicles, there has been a growing demand for tires further improved in steering stability and web grip performance.
In view of such circumstances, the inventor has conducted various researches and discovered that it is possible to further improve tires in rigidity balance at a tread section and achieve high-level compatibility between steering stability and wet grip performance, by setting a tire circumferential pitch P1 of the lateral grooves d1 arranged on the crown rib c1 as smaller than a tire circumferential pitch P2 of the lateral grooves d2 (inclined grooves e) arranged on the middle ribs c2 and larger than a tire circumferential pitch P3 of the lateral grooves d3 arranged on the shoulder ribs c3, and by defining the tire circumferential pitch P2 of the lateral grooves d2 and a circumferential length of the lateral grooves d2.